The invention lies within the field of reflectometry which consists, on the basis of the injection of one or more test signals into the cable or cable network to be tested, in measuring the reflected signal in the form of a reflectogram and in deducing therefrom an item of information on the discontinuities of impedances which are characteristic of electrical defects. In this manner it is possible to diagnose a complex wired network by detecting and locating possible defects.
The invention pertains in particular to a method for generating a multi-carrier reflectometry signal for implementation in a distributed-reflectometry system suitable for wired networks composed of numerous branches.
The invention also pertains to a method for diagnosing electrical defects on the basis of the injection of a multi-carrier reflectometry signal as well as a reflectometry device and a distributed system comprising a plurality of such devices.
The principle of reflectometry consists in injecting a signal onto a network of cables and then in measuring the echoes returned subsequent to the abrupt variation of the characteristic impedance of one or more transmission lines joined by connectors thus forming junctions. The characteristics of these echoes, such as the delay with respect to the test signal and the amplitude, make it possible to obtain information on the position and the type of the electrical defects present in this network.
In the case of on-line diagnosis, the test signal is dispatched over the network when the system to be diagnosed is in operation making it possible to experience the real conditions of the system and therefore to establish a more thorough diagnosis such as the characterization of intermittent or so-called transient defects. However, it is necessary to avoid any interference between the signals that are related to the operation of the target system and those injected by the reflectometry system. This is manifested by diverse constraints (varying from one application to another) as regards electromagnetic compatibility, mutual interference of the signals or robustness to noise.
This problem of interference is worse in the case of distributed diagnosis where several reflectometers perform a reflectometry measurement at several points of the target network simultaneously. The various signals which propagate in the network are liable to interfere mutually and thus to falsify the results of the diagnosis.
The field of reflectometry contains several methods which make it possible to respond, in certain cases of applications, to certain constraints of an on-line diagnosis. The following methods may be cited in particular.
SSTDR reflectometry (“Spread Spectrum Time Domain Reflectometry”), as described in the document referenced [1], is based on STDR reflectometry (“Sequence Time Domain Reflectometry”). It makes it possible to displace the spectrum of the signal emitted by way of the application of a modulation by a carrier frequency of the pseudo-random binary sequences of STDR reflectometry. However, the spectral occupancy is twice as large as with STDR reflectometry and the spectrum can just be shifted but not fully controlled.
NDR reflectometry (“Noise Domain Reflectometry”), as described in the document referenced [2], makes it possible to carry out reflectometry without emitting any signal. It is the signals already present on the line which are used. Although this method can be beneficial in certain cases, it exhibits major drawbacks: the signals present in the network of cables must have the appropriate properties, the processings are more complex and the test signals are not periodic, this having consequences on the complexity of the processing and on the quality of the measurement.
MCR reflectometry (“MultiCarrier Reflectometry”), as described in the document referenced [3], uses multi-carrier signals. Its benefit is the great flexibility with which the spectrum of the emitted signal can be modulated, thereby making it possible to accommodate constraints specific to on-line diagnosis. For example, if it is prohibited to emit on a frequency band situated in the middle of the spectrum of the test signal, it is entirely possible to cancel the energy of the signal on this band of frequencies. The proposed method remains, however, very limited since it allows only simple and uniform transmission lines to be diagnosed.
MCTDR reflectometry (“MultiCarrier Time Domain Reflectometry”) is also known, as described in the document referenced [4].
These methods of the known art suffer from a lack of flexibility in ensuring good on-line diagnosis in the case of a network of complex topology with the presence of non-straightforward defects. Knowing that the spectral occupancy of the test signal is one of the major aspects in ensuring good on-line diagnosis on the network of cables, the spectrum of the signal must make it possible to limit the interference with the operation of the application without degrading the quality of the reflectogram obtained.
In the case of a complex wired network, the detection and the location of one or more straightforward or non-straightforward defect(s) turn out to be impossible with a single injection point. Distributed reflectometry seems to be a good solution for remedying this problem. Distributed reflectometry is intended to mean the injection of the test signal at several points of the network and the recovery of the reflected signal at one or more points according to the chosen diagnosis strategy.
The document referenced [5] describes a reflectometry method, which consists in using pseudo-random sequences of M sequence type (or “Maximum Length Sequence”) as test signal and in minimizing the inter-correlation between the sequences generated by the various reflectometers. Thereafter, the post-processing (discrimination algorithm) consists in applying a suitable filtering as in the STDR method (“Sequence Time Domain Reflectometry”). The major drawbacks of this strategy are the following. On the one hand, it does not allow complete cancellation of the contribution of the other reflectometers. Residual noise remains. On the other hand, it requires the use of a signal of M sequence type (or LFSR, “Logical Feedback Shift Register” with maximum length). Indeed, in certain cases of application, test signals, such as multi-carrier signals, may be necessary on account of the constraints of the application concerned.
The document referenced [6] has proposed as recourse a method of selective averages which is totally independent of the test signal. The main idea is to insert weighting coefficients on emission of the signal and on reception when calculating the average. The choice of these coefficients is based on the Walsh-Hadamard sequences. This method makes it possible to obtain negligible levels of residual noise, but it turns out to be limited in the case of a complex network since the number of measurements increases in an exponential manner with the number of reflectometers in the network. In this case, the calculation of the selective averages demands a non-negligible calculation time in the case of several reflectometers, thus questioning the ability of the method to ensure real-time diagnosis, in particular as regards intermittent defects.